Hi GN Jha,
In order to burn dual layer DVDs, you will need a dual-layer capable optical drive. You can identify if your optical drive is dual-layer capable if it has any of the following labels:
Please check the below mentioned link for step by step instructions on how to burn a Dual Layer DVD disc using "Nero StartSmart":
A dual layer DVD is similar to the original DVD except it can store up to 8.5 gigabytes of data instead of just 4.7 gigabytes. You can also use a dual layer DVD for storing data or burn it as a data disk.
Please reply if you need any further assistance.